SA5008: check for missing closing quote

[nanzhong]

This PR attempts to address handling the missing closing quote case for the struct tag parser.

I could be naive here, but changing the current implementation of the scanner from breaking when it's unable to find the closing double quote to returning an error would achieve this. This does the change the behaviour to now ignore partially valid tags (i.e. `xml:"complete" json:"missing` would no longer result in the xml:"complete" being checked), instead it will highlight the missing closing quote violation.

I'm not familiar with the intended testdata organization, so I've proposed a test case for this under the example.com/CheckStructTags.

[arp242]

There's a bunch of other cases with quotes that should probably trigger an error; with your patch:

% git diff
diff --git i/staticcheck/sa5008/testdata/src/example.com/CheckStructTags/CheckStructTags.go w/staticcheck/sa5008/testdata/src/example.com/CheckStructTags/CheckStructTags.go
index 37b76813..966a6464 100644
--- i/staticcheck/sa5008/testdata/src/example.com/CheckStructTags/CheckStructTags.go
+++ w/staticcheck/sa5008/testdata/src/example.com/CheckStructTags/CheckStructTags.go
@@ -23,6 +23,14 @@ type T1 struct {
        O int        `json:"some-name,omitzero,omitempty,nocase,inline,unknown,format:'something,with,commas'"`
        P int        `json:"`          //@ diag(`missing closing quote`)
        Q int        `json:"some-name` //@ diag(`missing closing quote`)
+       R int        `json:some-name"`
+
+       AA int `json:"hmm""`
+       BB int `json:"hmm"""`
+       CC int `json:""hmm"`
+       DD int `json:"""hmm"`
+       EE int `json:""hmm""`
+       FF int `json:"""hmm"""`
 }

 type T2 struct {

% go test -count=1 ./staticcheck/sa5008
ok      honnef.co/go/tools/staticcheck/sa5008   0.578s

So no error on a missing starting quote or double quotes.

[nanzhong]

@arp242 you're totally right, however, the reason those are resulting in a pass is because the current implementation of parseStructTag intentionally silently stops parsing malformed struct tags when it encounters a part that does not follow the typical key:"value" format; note that all your test cases either are prefixed by a /json:".*"/ or do not start with a valid sequence.

I suspect the intent here is to be conservative since the language spec doesn't go beyond specifying that struct tags must be a string literal value. I'm not sure how strong @dominikh wants to preserve this behaviour. If not, then I can certainly update the struct tag parsing to fail more loudly, if so, then perhaps I could introduce a separate optional check that's more strict about the format of struct tags?